It’s complicated, like every thing else in Eastern Europe. Ukranians and Russians are the same ethnic group, for the most part. Ukraine is actally the original Russian homeland. Most Moldovans are essentially Romanians, non-Slavs. Transnistria is a predominately Slavic part of Moldova that indentifies with Russia.
Ukraine is to Russia what the South/Confederacy are/were to America, fast forwarded a few hundred years.
What I don’t understand is how the Russian troops in Transnistria are sustained. Ukraine, which is at the brink of war with Russia, almost certainly will not allow Russian planes to overfly its territory. The only other approach would be to overfly Romania and Moldova.